1. Field of the Invention
The present invention relates generally to centrifugal chillers, and more particularly, to a method and apparatus for applying dual centrifugal compressors to a refrigeration chiller unit.
2. Description of the Related Art
As shown in FIG. 1, a conventional centrifugal chiller 10 includes a single centrifugal compressor 12, a condenser 14, an expansion device 16, and an evaporator 18, all being interconnected in series to form a conventional closed refrigeration circuit. Compressor 12 compresses refrigerant gas and delivers it to condenser 14 where a cooling medium, such as water from a cooling tower, causes the compressed gas to condense to a liquid refrigerant. The liquid refrigerant expands as it is passed through expansion device 16 and travels to evaporator 18. As the liquid refrigerant flows through evaporator 18, circulating water from a building is placed in a heat exchange relationship with the refrigerant so as to cause the water to be chilled and the refrigerant to be vaporized. The refrigerant is then delivered to a suction inlet of the compressor. In this manner, the water is chilled in evaporator 18 for cooling the building. In order to vary the amount of cooling imparted to the building in response to changes in the cooling requirement or load, the capacity of compressor 12 is adjusted, thereby regulating the flow rate of the refrigerant through the refrigeration circuit.
Conventionally, horizontal shell-and-tube condensers are used in centrifugal chillers, often with the refrigerant flowing outside the tubes of the condenser. As generally illustrated in FIG. 2, a plurality of tubes 20 are held within a cylindrical shell 22 of the conventional condenser. The outside surface 24 of each tube 20 is often finned to increase the heat transfer of the refrigerant outside the tubes with water 26 running through the tubes. Furthermore, condensers usually have the cylindrical shell like that shown in FIG. 2.
In order to increase the capacity of centrifugal chiller systems, it has been proposed to provide a refrigeration system with dual compressors. Such systems are discussed in U.S. Pat. No. 4,201,065 to Griffen, and one such system is described in some detail in that patent. Aspects of a prior art dual compressor system are shown in FIG. 3. Compressors 28, 30 are connected to separate circuits in condenser 32. The system includes, expansion valves 36, 38 provided in the lines connecting condenser 32 and evaporator 34.
Condenser 32 includes a plurality of separate and distinct refrigeration tube circuits 40, 42, 44 and 46. Each such circuit includes a plurality of parallel elongated tubes (not shown) that run the length of the condenser and are interconnected by bends at the ends of the condenser. Each circuit in Griffen includes parallel tubes (not shown) extending along the length of condenser 32. Circuits 40 and 42 both transverse the top half of the condenser 32, and circuits 44, 46 transverse the lower half of condenser 32. Compressor 28 therefore feeds refrigerant through a circuit 42 in the top half of the condenser and circuit 46 in the lower half of the condenser. Compressor 30 similarly feeds refrigerant to circuits in both the top and bottom of the compressor. If one compressor is shut off when the refrigeration load decreases, or if the flow of refrigerant through one or more circuit is otherwise cut off, the condenser still utilizes the heat exchanger area of the entire condenser.
In the system shown in Griffen, the refrigerant flows through tubes in the condenser and is cooled by a fluid, typically air, that is placed in heat exchange relationship with the refrigerant to cool it. Similarly, the refrigerant flows through tubes in the evaporator where it is used to cool the water flowing through the evaporator shell.
Conventional dual compressor refrigeration systems typically use conventional positive displacement compressors, such as, reciprocation or screw-type compressors. Conventional positive displacement compressors operate in parallel and have common suction and discharge connections. Centrifugal compressors have variable volume and constant head characteristics, and thus, cannot operate in parallel, unless balanced through control. Thus, in order for centrifugal compressors to be effectively implemented in such dual-compressor systems, a control system must be employed that suitably matches the head characteristics for each centrifugal compressor. However, in practice such head matching is difficult to achieve even with the most sophisticated control systems.